Programmable pulse electroplating process

ABSTRACT

An improved electroplating process is achieved by pulsing the current through an electrolyte containing a chromium plating solution generating hydrogen at the surface of a workpiece while simultaneously performing honing operation, to enable the hydrogen to escape intermittently during the reduced current periods to avoid the build-up of stress and to provide a softer plating coating adjacent to the workpiece, and, to thereafter, decrease the duration of the reduced current periods to retain the hydrogen and thus to increase the hardness of the outer layers of the plating where necessary to provide a heavy duty wearing surface.

United States Patent [191 Leland PROGRAMMABLE PULSE ELECTROPLATING PROCESS [76] Inventor: James M. Leland, 4591 Mt.

LaPlatta PL, San Diego,'Calif. 921 17 22 Filed: Nov. 1, 1973 211 App]. No.: 411,962

[52] US. Cl. 204/36; 204/26; 204/51; 204/228; 204/DIG. 9; 204/DIG. 10; 29/194;

[51] Int. Cl. C23b 3/06; C23b 5/50; C23b 5/56 [58] Field of Search 204/25, 26, 36, 51, 218, 204/228, DIG. 8, DIG. 9, DIG. 1O

[56] References Cited UNITED STATES PATENTS 2,421,863 6/1947 Beck 204/41 X 2,706,170 4/1955 Marchese 204/3 3,042,592 7/1962 Schaer 204/51 3,232,854 2/1966 MacLean et a1 204/51 3,318,786 5/1967 Faust et al. 204/51 3,437,568 4/1969 Hasselmann et a1. 204/3 3,492,544 1/1970 Falanga 204/228 X 3,616,289 10/1971 Ellis et a1. 204/26 3,753,664 8/1973 Klingenmaier.. 29/191 3,804,725 4/1974 Haynes 204/26 [4 1 May 27, 1975 FOREIGN PATENTS OR APPLICATIONS OTHER PUBLICATIONS Elcctroplating Chromium Directly on Aluminum By C. L. Faust, et al., Plating Magazine, June 1961.

Primary Examiner-Howard S. Williams Assistant ExaminerAaron Weisstuch Attorney, Agent, or FirmR. S. Sciascia; G. .I. Rubens [57] ABSTRACT An improved electroplating process is achieved by pulsing the current through an electrolyte containing a chromium plating solution generating hydrogen at the surface of a workpiece while simultaneously performing honing operation, to enable the hydrogen to escape intermittently during the reduced current periods to avoid the build-up of stress and to provide a softer plating coating adjacent to the workpiece, and, to thereafter, decrease the duration of the reduced current periods to retain the hydrogen and thus to increase the hardness of the outer layers of the plating where necessary to provide a heavy duty wearing surface.

5 Claims, 1 Drawing Figure PULSE PLAT/NG POWER SUPPLY 3/1945 United Kingdom 204/41 I PAIENTED MAY 27-1915 I PULSE PLAT/N6 POWER SUPPLY 34 @3 (D TIMER PROGRAMMER 0N OFF TIMERS PROGRAMMABLE PULSE ELECTROPLATING PROCESS STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION This invention relates to a method and article of electroplating, designed primarily for electroplating hard metals such as chromium by means of a pulse-honing technique, but it is to be understood that the method can be used for any purposes for which it is found applicable.

The hone-forming process in the electroplating art is utilized in the industrial plating of cylinders, plates, and the like requiring a very hard outer work surface. This prior art process involves the simultaneous and combined operations of plating and machining, or mechanically working, the surface to be plated by a lapping or honing tool. The plating operation is conducted continuously with a steady DC current passing through a hard metal alloy electrolyte, such as chromium. The honeforming process is described in US. Pat. Nos. 3,022,232; 3,183,176; and 3,637,469. The advantages of this type of plating process are several. Firstly, the build-up of the hard metal, usually chromium, is consistent throughout workpiece surfaces contacted by the hones filling in all the irregularities and providing a very smooth, accurate, and bright surface. Secondly, the process employs a high current density resulting in a high deposition rate, up to fifty times faster than the conventional electroplating tank process. The honeforming process is also more expeditious, up to 3 mils thickness/minute, in that numerous rinsing and etching steps in the conventional plating process can be omitted. Consequently, the hone-forming process is more suitable to automatic production lines with a consequent savings of time and material. Thirdly, and most important is that the honing-plating technique pro duces an extremely hard surface achieved without special solutions or additions. Various tests have indicated that in hone-formed chromium plating a hardness of on the average of 1200-1400 Knoop can be obtained, as compared to 700-800 Knoop with conventional tank electroplated claddings. The higher hardness of the hone-forming process has been attributed to the mechanical work introduced in the plating process by the honing operation. It should be noted that in the honeforming method the extreme hardnessexists uniformly through the thickness of the plating.

The high degree of hardness offered by the honeplating process has broad areas of applications, such as in original manufacturing of pumps, connecting rods, and especially in the reconditioning of worn surfaces in such components.

Although the combined hone-forming electroplating process offers numerous and significant advances, there is one serious and inherent disadvantage that naturally flows from thisv process. The high hardness existing throughout the thickness of the plated metal by the hone-forming process has locked-in residual tensile stresses adversely affecting the junction between the base metal and the plating causing adhesion failures.

This can be primarily attributed to hydrogen embrittlement.

As will be described, the novel method of this invention solves the problem of excessive hardness, relieves the build-up of tensile stresses, and avoids hydrogen embrittlement. Thus, the invention process improves the hone-forming technique so as to have practical significance heretofore not achieved in the art.

SUMMARY OF THE INVENTION It has been discovered that the adhesion problems in the high hardness plating obtained with the honeforming technique can be greatly reduced by variably controlling the plating current, usually a DC source. This current control consists of providing on" and of current periods by pulsing the plating current, instead of relying on the continuous steady current that is presently utilized in the hone-forming process. Of paramount importance in the invention process, is that the variable control of the current consists in varying the off-time current periods, and more specifically by decreasing the duration of the off-time periods to obtain a coating of graduated hardness.

It should be noted that the technique of uniformly fluctuating a plating current in the conventional electroplating of chromium on aluminum has been employed for many years in the ornamental plating art for plating household items such as tablewear, decorative bowls, etc. The purposes of this technique are to obtain a bright, decorative, corrosion resistant plating which is softer and thinner than decorative coatings obtained by the conventional tank plating methods. The softer and brighter chromium coating having a hardness approximately of 400 Knoop makes it more economical to polish by skilled workmen by the conventional buffing operation. In addition, the lesser hardness of the coating, produces a crack-free coating which is highly corrosive resistant. This enables a thinner coating to be utilized. It is obvious that these attributes materially reduce plating and buffing times, with a savings of plating materials, and contribute to a material savings in manufacturing cost of plating ornamental products.

However, the purpose in the invention method of pulsing the steady direct current in the hone-forming process, is to provide a soft bonding plating only at the junction of the plating with-the workpiece to reduce the tensile stress that is inherently formed in the high hardness coatings obtainable in the hone-plating process. This is an important consideration since the current products plated with the hone-forming process inherently require in their use a very high hardness surface. Interrupting the plating currentt in the hone-forming process as proposed by the present invention provides deposition periods or on time and non-deposition or off-time periods. During the deposition period in chromium plating, it is important that only one layer maximum of the chromium hydride (CrH is deposited on the base metal. Duringthe subsequent nondeposition period, the CrI-I,, being thermally unstable, is afforded time to decompose and the hydrogen gas is allowed to escape before the commencement of the next deposition period.

Thereafter, as the plating method progresses the pulsed power source is programmed so that the nondeposition, or off-time, period is gradually reduced in duration to increase the hardness of the subsequent plating. It should be noted that during the programming of the offtime" the on-time preferably remains constant. This gradual reduction of off-time" continues until near the end of the process a zero off-time may be reached at which time the maximum hardness is obtained where needed at the wearing surface.

Thus, it is an important aspect ol tlllS invention to in troduce in the hone-forming electroplating process a pulsing of the current. and equally important a pulsing at a variable rate to achieve a variable hardness without which the objects of this invention could not be achieved.

STATEMENT OF THE OBJECTS OF THE INVENTION It is a principal purpose of this invention to provide a method for producing a hard metal alloy electroplating in which the tensile stresses and adhesion failures are minimized.

Another important object is to provide such a method for electroplating a hard metal alloy containing hydrogen which provides a plating having a minimum of hydrogen embrittlement.

Still another important object is to achieve the foregoing results by producing a hard metal alloy electroplating having a variable hardness in the plating thickness; and a corollary object is to provide such variable hardness gradient with the least hardness being at the juncture between the coating and the workpiece.

BRIEF DESCRIPTION OF DRAWINGS The single FIGURE is a diagrammatic illustration of typical equipment which may be employed in the novel programmable pulse hone-forming method.

PREFERRED EMBODIMENT Referring to the drawing, there is shown a diagrammatic view of the hone-forming equipment that can be employed in practicising the novel method of this invention.

The hone-forming equipment may be of conventional, off-the-shelf design including a fixture 12 for supporting a workpiece I4, such as a cylinder or the like have a core surface to be electroplated. A honing tool 16 having a plurality of circumferentially spaced honing stones 18 is adapted to be extended into the bore of the workpiece to mechanically work the surface simultaneously during the plating operation in a manner well known in the art as hone-forming. Hon ing tool 16 is supported by a spindle 20, which in the case of a cylindrical work surface is given a rotary and reciprocal motion in a manner illustrated in any of the aforementioned patents, and need not be illustrated. Although a cylindrical workpiece is illustrated in the preferred embodiment, it is understood that flat or other configured surfaces can be plated employing the invention process, and in such case the honing tool would have a conforming configuration.

Tank 22 contains a plating solution 24 which is circulated between the honing tool and the surface to be plated by a pump 26 and suitable piping. For hard metal industrial plating of bearing surfaces or the like, the electrolytic solution is usually a chromic acid.

An electric power supply 28 provides DC power for the electroplating operation. Power is connected to honing tool 16, the anode, by conductor 30 through a suitable slip ring arrangement (not shown) and to workpiece 14, the cathode, by conductor 32. For purposes of this invention, as will be described later, power supply 28 must be a pulse-type rectifier having variable on-time controls and off-time" controls. Such a pulse plating power supply is commercially available, from the Chem-O-Tronic Inc. of Justice, lll. under Chem-O-Plate" Model 1000.

The following description of the novel programmable pulse-honing process is applicable to any hard metal alloy plating electrolyte containing hydrogen ions, the most common electrolyte used for heavy industrial application being conventional chromium plating solution which will be used hereafter in the discussion as a typical plating solution.

The prior art hone-forming method of marrying the electroplating process and the hone-forming process is able to produce a substantially harder and heavier plating at a very high plating rate, and therefore offers many advantages in commercial production. However, as has been previously mentioned, the extreme hardness obtained by the hone-forming process inherently brings about a critical problem in that it causes a high residual tensile stress resulting in adhesion failures primarily due to hydrogen embrittlement. This characteristic is very detrimental in heavy commercial plating applications, such as cylinders and other bearing surfaces.

It is believed that this problem can best be appreciated by discussing the phenomena which occurs in the chromium deposition when applied to the honeforming method. The chromium hydride (CrH electrodeposited on the cathodic workpiece is thermally unstable and, upon deposition, commences to decompose and shrink into chromium metal and hydrogen atoms. Continuous deposition by the steady DC current employed in hone-forming, especially at the high deposition rate that is characteristic of this process, prevents the decomposition of the chromium hydride as the hydrogen ions have no opportunity to escape. This condition is believed to cause shrinkage crack patterns to occur in the deposited layers, the build-up of tensile stresses, and hydrogen embrittlement when occurring adjacent to the base metal being plated.

The present invention utilizes a pulsed current to provide alternate deposition, or on-time, periods and non-deposition, or off-time periods, in contrast to the steady continuous current utilized in the conventional established hone-forming method. For reasons presently to be described, it is important in the present invention that the on-time plating periods be set so as not to exceed that time necessary to deposit one complete monolayer of CrH molecular units. And that the off-time periods be initially programmed to allow sufficient time for the CrH, in contact with the plating solution to decompose, and the hydrogen atoms in the form of molecular gaseous hydrogen to escape before the next layer of CrH, is deposited. In other words the previously deposited coating of CrH, is allowed to decompose, or to partially decompose, before the next layer of Crl-I, is deposited thereover. This technique will produce a softer plating.

It is important to note that decomposition of the CrH can not occur if the plating current is on continuously, as in the prior hone-forming method relying on a steady DC power, and thus the hydrogen gas is unable to escape and remains occluded in the coating to cause tensile stresses.

The optimum value of on-time" period of the plating current, assuming a square pulse waveform, can be determined by the following equation:

where:

Q the quantity of CrH, molecules required to cover one square inch of surface evenly and completely with one layer of CrH, molecules 1.2 X l units/sq.in.

1 unit Wmp-sec (where Cr valence 6) CD Instantaneous true maximum current density in (square wave assumed) Maximum t .0115 sec or 11.5 milliseconds Having thus determined the duration of the maximum desired on-time periods for the plating current, the present invention proposes to vary the duration on the off-time periods to achieve the novel results. Accordingly, the hardness of the chromium deposits may be varied by controlling the length of the off-time periods, which in turn regulates the amount of hydrogen gas that is allowed to escape, between the limits of 100% hydrogen escape producing fully soft chromium, and 0% hydrogen escape producing fully hard chromium. The latter limit is always achieved with the con ventional hone-forming method.

The length of the "off-time periods can be controlled manually by timers 34 or automatically by timer programmer 36 both associated with power supply 28. The programming of any specific plating operation will depend on many variables. Generally speaking, for industrial plating of heavy wearing surfaces the plating thickness may vary from 0.002 to 0.050 inch. Assuming a desired plating thickness of 0.020 inch, and a plating rate of one hour for each 0.010 inch plating thickness,

and 50% on-time and off-time" periods, which practically speaking may be considered suitable starting plating conditions, with an on-time" of 1 1.5 milliseconds; the duration of the off-time period would be reduced for example by one millisecond approximately every 6 minutes during the process after an appropriate delay while forming the initial fully soft deposits. Before the final 0.003 to 0.008 inch plating thickness is reached, the off-time period is set to be zero or the desired minimum off-time. Thereafter,

the process usually amounts to a straight non-pulsed current with maximum hardness or the desired controlled hardness achieved at the outer plating surface.

An investigation of the honing process discloses that in a typical operation the vertical velocity of the honing tool is of a low order compared to a typical off-time pulse period, and, therefore, there is no need to coordinate the electrical pulses with respect to the mechanical position of the honing tool.

The novel invention method provides a significant and important improvement in the conventional honeforming electroplating art giving practical utilization to its many advantages, especially in heavy plating appli-,

cations. The build-up of stresses in the plating is avoided by allowing the hydrogen ions to escape during the plating process. This is accomplished by programming the plating current with on-time" and off-time" current pulses of predetermined durations. That is to say, that the chromium hydride that is deposited during an on-time current pulse is able to decompose or partially decompose in a controllable manner during the subsequent off-time" current period, allowing the hydrogen to escape or to partially escape as desired before the next on-time plating current pulse. This results in an initial base plating of soft chromium that is not subjected to hydrogen embrittlement.

A significant step of the invention process is to gradually decrease the duration of the off-time pulses in order to gradually increase the hardness of the chromium plating, so that the outer workpiece surface will have the hardness that will satisfy the requirements in the particular application while avoiding hydrogen embrittlement of the base metal, and tensile-stressadhesion failures of the plating.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is:

1. The method of controlling the plating hardness in a hone-forming process in which the surface of a metal workpiece is cathodically electroplated in an electrolytic chromium plating solution generating hydrogen at the workpiece surface, with a tool having a honing surface movable relative to and in engagement with the workpiece comprising the steps of:

applying a DC current through said electrolytic solution; interrupting the flow of current through said solution to provide alternate deposition periods and nondeposition periods of predetermined duration;

gradually decreasing the duration of the nondeposition periods during the plating process;

whereby during the deposition period a layer of chromium is deposited on the work surface simultaneously with the honing operation, and during the non-deposition period a predetermined amount of the hydrogen is allowed to escape to control the hardness of the chromium applied to the surface of the workpiece and to reduce the stress between the plating and the workpiece, the hardness of the plating increasing as the duration of the non-deposition period decreases.

2. The method of claim 1 wherein the duration of the deposition periods is substantially constant throughout the plating process.

3. The method of claim 2 wherein the deposition period is not substantially longer than the time necessary to deposit one complete monolayer of the chromium.

4. The method of claim 1 wherein the non-deposition periods are gradually decreased to approach zero during the progression of the plating process to obtain the maximum hardness desired on the outer plating surface.

5. The method of claim 1 wherein the electroplating process utilizes only one plating bath. 

1. A METHOD OF CONTROLLING THE PLATING HARDNESS IN A HONEFORMING PROCESS IN WHICH THE SURFACE OF A METAL WORKPIECE IS CATHODICALLY ELECTROPLATED IN AN ELECTROLYTIC CHROMIUM PLATING SOLUTION GENERATING HYDROGEN AT THE WORKPIECE SURFACE, WITH A TOOL HAVING A HONING SURFACE MOVABLE RELATIVE TO AND IN ENGAGEMENT WITH THE WORKPIECE COMPRISING THE STEPS OF: APPLYING A DC CURRENT THROUGH SAID ELECTROLYTIC SOLUTION; INTERRUPTING THE FLOW OF CURRENT THROUGH SAID SOLUTION TO PROVIDE ALTERNATE DEPOSITION PERIODS AND NON-DEPOSITION PERIODS OF PREDETERMINED DURATION; GRADUALLY DECREASING THE DURATION OF THE NON-DEPOSITION PERIODS DURING THE PLATING PROCESS; WHEREBY DURING THE DEPOSITION PERIOD A LAYER OF CHROMIUM IS DEPOSITED ON THE WORK SURFACE SIMULTANEOUSLY WITH THE HONING OPERATION, AND DURING THE NON-DEPOSITION PERIOD A PREDETERMINED AMOUNT OF THE HYDROGEN IS ALLOWED TO ESCAPE TO CONTROL THE HARDNESS OF THE CHROMIUM APPLIED TO THE SURFACE OF THE WORKPIECE AND TO REDUCE THE STRESS BETWEEN THE PLATING AND THE WORKPIECE, THE HARDNESS OF THE PLATING INCREASING AS THE DURATION OF THE NONDEPOSITION PERIOD DECREASES.
 2. The method of claim 1 wherein the duration of the deposition periods is substantially constant throughout the plating process.
 3. The method of claim 2 wherein the deposition period is not substantially longer than the time necessary to deposit one complete monolayer of the chromium.
 4. The method of claim 1 wherein the non-deposition periods are gradually decreased to approach zero during the progression of the plating process to obtain the maximum hardness desired on the outer plating surface.
 5. The method of claim 1 wherein the electroplating process utilizes only one plating bath. 